The invention relates to a high-speed lock nut, especially for attaching a vehicle wheel to the clamping shaft of a balancing machine. The nut contains radially movable threaded segments to permit disengaging the threads of the nut from the threads of the clamping shaft to move the nut quickly up the shaft into contact with the vehicle wheel. Thereafter, the segments automatically close to engage the threads and the nut is tightened to final locking position.